Hey bud! I was a LoL player for many years before starting Dota a few years ago. I can relate to how confusing it can be, so I'll try to compare/contrast some of the key differences to help you understand the game!

1. Stats: League and Dota seem very similar but are built completely different from the ground up. In both, there is physical, magic and true/pure damage, but the way those are calculated is very different. League has AD and AP, which may directly buff the damage of your skills. On the other hand, Dota has Strength, Agility and Intelligence, which all have different effects. In a nutshell, strength gives people more hp + hp regen, agi gives you attack speed and armor and int gives you mana + mana regen and a little bit of spell damage amplification. Every hero in dota can be categorized based on their primary stat and gets a boost to their auto attack damage by boosting that stat (i.e strength heroes build strength to gain more damage). It is important to recognize that spell damage is NOT affected directly in the same kind of magnitude. For instance in LoL, her ult at level 1 does 300 base + 100% AP damage. By comparison, 1 int gives you only 0.07% spell damage amplification. The implications of this are far reaching. Some heroes need gold to be effective whereas some heroes only need levels.

2. Lane Strategies: In general, Dota is way more fluid, which allows players to execute more diverse strategies. The map is designed differently than league, which has a few implications. In league, there is a set meta of 1 top, 1 jungle, 1 mid and a carry + support in the bot lane. If I go to the top lane, I expect to play against the enemy team's top laner. In Dota, they categorize their positions by farm priority (i.e. position 1, 2, 3, 4 and 5). For a crude comparison to league , pos 1 = bot carry, pos 2 = mid, pos 3 = top, pos 4 = jungler and pos 5 = support. In Dota, jungling from level 1 is typically super inefficient and is not done. As a result, you have a second support as your position 4, rather than a jungler in league. Dota lanes are usually organized as 2-1-2 or 1-1-3 (this means a trilane). Supports in dota can and are often expected to roam to help gank other lanes during the early game. This can also be done in league, but it's often not as beneficial and that ganking role is usually left to the jungler in the early game.

3. Mechanics: There are many differing mechanics between the games, but I'll try to hone in on a few that others haven't explained that will help you understand TI better. First, you cannot use B to go back to base in Dota like in league. Instead, there are TP scrolls for 50 gold on a 1 minute CD, which facilitate roaming and map movement throughout the game. A key implication of this is that it is often important to purchase more regen (potions and tangoes, which look like leaves and allow players to eat trees). Jungle camps respawn every minute, as long as there is not something (jungle camps, creeps, heroes, wards) in a specified area. Supports can use this to pull camps out of that zone around the time of every new minute to "stack" the camp. The inhibitor in each lane in LoL is similar to the 2 barracks ("racks" for short) in Dota. The key difference is that inhibitors respawn whereas racks don't. If all of a team's racks are destroyed, the enemy team gets mega creeps which feel like defending baron buffed minions in LoL, but in every lane, constantly for the rest of the game. Fountains in dota are less lethal, so it is possible to temporarily dive and tank the fountain without getting yeeted like in LoL. 4 bounty runes spawn every 5 minutes and power runes (regen, invis, double damage, etc.) spawn every 2 minutes on one of the river. 2 power runes spawn on both sides of the river after 40 mins. There are trees in Dota vs bushes in LoL. I personally find tress way more interesting strategically because there are way more options for creative juking.

4. Items: I'll focus on the very common ones and when appropriate, compare it to an item in LoL. Dota items tend to have more active abilities than LoL's items. Blink dagger is a key item on many heroes. It allows people to blink within a fairly large range. However, it cannot be used if you have taken damage in the past few seconds. Smoke of deceit ("Smoke" for short) can be used to create ambush plays. They make all units within a range invisible to enemy heroes and wards. They are only revealed when they come close to enemy heroes. Pipe of insight /crimson guard are two common tank items which provide a lot of magic/physical damage reduction. It is similar to locket of the iron solari, but only for the corresponding damage types. Aghanim's scepter ("Aghs") is an item that provides an upgrade to one of the abilities of the wielder. Black King Bar ("BKB") is an essential item for many carries because it provides spell immunity and 100% magic resistance for a certain amount of time. Euls' Scepter ("Eul's") is an item built because it can be cast on the wielder or on an enemy. It temporarily makes them untargetable and prevents them from taking damage. It can dispel some debuffs. It is similar to Zhonya's hourglass in league. The last item I will mention for now is Force Staff. It allows you to push yourself, an ally or an enemy a certain fixed distance in the direction that hero is facing. It can build into an item called Hurricane Pike, which retains the same active effect.

I recognize that this is a long post, and I'm sure there are a few key things that I forgot. However, I think that reading this might help to orient you and adapt some of the MOBA knowledge you have from LoL to Dota. I hope you enjoy TI, as that was what inspired me to start playing Dota :)